Some 30% of all proteins are predicted to be found in a cellular membrane. But in contrast to the many structures of soluble proteins known, only a handful of membrane protein structures have been determined. We are working to crystallize and determine a structure by X-ray crystallography of a membrane protein. We are trying to produce and to crystallize sufficient amounts of several proteins: one or more chemokine receptors, members of a large family of homologous G-protein linked receptors; one or more tetraspan family members, important in the immune system; and the transporters associated with antigen processing (TAP). Our approach is to produce the proteins in bacteria as the denatured polypeptide chains that bacteria often can produce and then to renature the proteins to their native and functional conformations. After obtaining enough of the pure protein, we will crystallize the protein in preparation for the X-ray crystallographic structure determination.